1. Field of the Invention
The present invention relates to an inorganic board that is excellent in crack resistance and fire resistance and is preferably used as a building board, and a method for manufacturing the inorganic board.
2. Description of the Related Art
Conventionally, inorganic boards having excellent physical properties such as strength and having a good productivity have been manufactured by forming sheets using an apparatus such as a Hatschek sheet forming machine or a flow-on sheet forming machine from a slurry in which a hydraulic material is dispersed in water, and dehydrating and then curing the sheets. In order to improve physical properties of such inorganic boards, such as crack resistance, fire resistance, and dimensional stability, wollastonite is in some cases added to and mixed with the slurry.
For example, JP 2000-301511A discloses a method for manufacturing a fiber reinforced cement board, including the step of forming a sheet from a cement slurry, wherein wollastonite in an amount of 0.5 to 5 wt % with respect to the total solid content weight of the cement slurry is dispersed in a green sheet immediately after the supply thereof on a filter cloth, and the green sheet is then vibrated. However, since the performance of wollastonite exerted varies depending on the pulverization method or the classification method, wollastonite of a proper grade has to be used such that the inorganic board is made, for example, crack resistant, fire resistant, and dimensionally stable. Unless wollastonite of a proper grade is used, the flexural strength may be lowered, and the productivity may deteriorate due to water filtering characteristics and the like, for example.
JP H7-166644A discloses a fiber cement board, containing cement, a pulp fiber material, and an aggregate, and further containing an inorganic additive having an aspect ratio of 10 or more, wherein the inorganic additive is wollastonite. However, since wollastonite is in the form of needle-like crystals having various sizes, even when wollastonite has an aspect ratio of 10 or more, the inorganic board may not be, for example, crack resistant, fire resistant, or dimensionally stable, depending on the size distribution of the wollastonite. Furthermore, if wollastonite contains a large amount of fine crystals, the flexural strength may be lowered, and the productivity may deteriorate due to water filtering characteristics and the like. Accordingly, the aspect ratio is not reliable as a standard in selecting wollastonite.